Fluorescence enhancement of Tb3+ in Tb aromatic acid complexes: Correlation of synergistic enhancement with the structure of the ligand

Fluorescence enhancement of Tb3+ has been studied, using aromatic carboxylic acids as ligands. These ligands enhance the fluorescence of Tb3+ by about three orders of magnitude. The enhancement is observed to be due to ligand sensitized fluorescence. The fluorescence of Tb3+ in some of these complexes is further enhanced by an order of magnitude by the addition of trioctyl phosphine oxide (TOPO), a phenomenon referred to as synergism. However, the pattern of synergism displayed by TOPO is not uniform, and is found to vary with the ligand. While with some aromatic acids, TOPO displayed maximum synergism near pH 6, in others it did so around pH 4. In a few cases, TOPO did not display any synergism at all, at any of the pH values. These observations are discussed in detail and the results are rationalized on the basis of the structure of the aromatic acids.